This disclosure relates to the field of visual displays. In particular, it relates to visual displays including two-particle electrophoretic systems having a controlled response to tribo-electric charging effects. More particularly, this disclosure relates to large area visual displays including two-particle electrophoretic systems having a controlled response to tribo-electric charging effects.
Paper has traditionally been a preferred medium for the presentation and display of text and images. Paper has several characteristics that make it a desirable display medium, including the fact that it is lightweight, thin, portable, flexible, foldable, high-contrast, low-cost, relatively permanent, and readily configured into a myriad of shapes. It can maintain its displayed images without using any electricity. Paper can also be read in ambient light and can be written or marked upon with a pen, pencil, paintbrush, or any number of other implements, including a computer printer.
Unfortunately, paper is not well suited for large-area or real-time display purposes. Real-time imagery from computer, video, or other sources cannot be displayed directly with paper, but must be displayed by other means, such as by a cathode-ray tube (CRT) display or a liquid-crystal display (LCD). However, real-time display media lack many of the desirable qualities of paper, such as stable retention of the displayed image in the absence of an electric power source.
Electric paper combines the desirable qualities of paper with those of real-time display media. Like ordinary paper, electric paper can be written and erased, can be read in ambient light and can retain imposed information in the absence of an electric field or other external retaining force. Also like ordinary paper, electric paper can be made in the form of a light-weight, flexible, durable sheet that can be folded or rolled into a tubular form about any axis and placed into a shirt or coat pocket, and then later retrieved, re-straightened, and read without loss of information. Yet unlike ordinary paper, electric paper can be used to display full-motion and other real-time imagery as well as still images and text. Thus, electric paper can be used in a computer system display screen or a television.
Traditionally, electronic displays such as liquid crystal displays have been made by sandwiching an optoelectrically active material between two pieces of glass. In many cases, each piece of glass has an etched, clear electrode structure formed using indium tin oxide (ITO). A first electrode structure controls all the segments of the display that may be addressed, that is, changed from one visual state to another. A second electrode, sometimes called a counterelectrode, addresses all display segments as one large electrode, and is generally designed not to overlap any of the rear electrode wire connections that are not desired in the final image. Alternatively, the second electrode is also patterned to control specific segments of the display. In these displays, unaddressed areas of the display have a defined appearance.
Electrophoretic displays offer many advantages compared to liquid crystal displays. Electrophoretic display media are generally characterized by the movement of particles through an applied electric field. Encapsulated electrophoretic displays also enable the display to be printed. These properties allow encapsulated electrophoretic display media to be used in many applications for which traditional electronic displays are not suitable, such as flexible displays. Additionally, electrophoretic displays typically have attributes of good brightness, wide viewing angles, high reflectivity, state bistability, and low power consumption when compared with liquid crystal displays. However, problems with the image quality, specifically the contrast, to date has been less than optimal. Contrast is defined as the ratio of the white state to the dark state reflectance of the display. Contrast enables the eye to easily distinguish between light and dark.
The gyricon, also called the twisting-ball display, rotary ball display, particle display, dipolar particle light valve, etc., provides a technology for making electric paper and electrophoretic displays. A gyricon display is a display that can be altered or addressed. A gyricon display is made up of a multiplicity of optically anisotropic balls which can be selectively rotated to present a desired surface to an observer.
The optical anisotropy of the gyricon balls is provided by dividing the surface of each gyricon ball into two or more portions. One portion of the surface of each gyricon ball has a first light reflectance or color. At least one other portion of the surface of the gyricon ball has a different color or a different light reflectance. For example, a gyricon ball can have two distinct hemispheres, one black and the other white. Additionally, each hemisphere can have a distinct electrical characteristic, such as, for example, a zeta potential with respect to a dielectric fluid. Accordingly, the gyricon balls are electrically as well as optically anisotropic. It is conventionally known that when particles are dispersed in a dielectric liquid, the particles acquire an electric charge related to the zeta potential of their surface coating.
The black-and-white gyricon balls are embedded in a sheet of optically transparent material, such as an elastomer layer, that contains a multiplicity of spheroidal cavities. Each of the spheroidal cavities is permeated by a transparent dielectric fluid, such as a plasticizer. The fluid-filled cavities accommodate the gyricon balls, one gyricon ball per cavity, to prevent the balls from migrating within the sheet. Each cavity is slightly larger than the size of the gyricon ball so that each gyricon ball can rotate or move slightly within its cavity.
A gyricon ball can be selectively rotated within its respective fluid-filled cavity by applying an electric field, so that either the black or white hemisphere of the gyricon ball is exposed to an observer viewing the surface of the sheet. By applying an electric field in two dimensions, for example, using a matrix addressing scheme, the black and white sides of the balls can be caused to appear as the image elements, e.g., pixels or subpixels, of a displayed image.
Conventional gyricon displays are described further in U.S. Pat. Nos. 4,126,854; 4,143,103; 5,389,945 and 5,739,801 to Sheridon, the disclosures of which are incorporated herein in their entirety. Gyricon displays can be made that have many of the desirable qualities of paper, such as flexibility and stable retention of a displayed image in the absence of power, that are not found in CRTs, LCDs, or other conventional display media. Gyricon displays can also be made that are not paper-like, for example, in the form of rigid display screens for flat-panel displays.
However, electronic papers such as gyricon displays are not necessarily compatable with low resolution applications and large area, outdoor electronic signage. In order to achieve a gyricon contrast ratio of about 8 and a white reflectivity of from 20–22% up to 28% requires the gyricon beads have a 90–95% perfect bichromality, the distinct equatorial separation of black and white hemisphere. In addition, the color axis and dipole moment axis of the beads must be aligned, and all the beads must complete their rotation. In addition, the layer of spherical capsules in which the beads are located is a major contributor to optical reflectivity, but hexagonal close packed spheres only occupies 90.7% of this layer. Gyricon devices are usually made into a multilayer configuration to enhance optical performance. For high-brightness large-area visual displays, 100% coverage of reflective surface is desired. These requirements pose significant challenges in low resolution applications and large area electronic signage.
One example of electrophoretic displays being developed to address the challenges of low resolution applications and large-area electronic signage involves an electrophoretic ink that uses spherical cells or microcapsules filled with black and white particles. The particles can be manipulated to position themselves on the top or the bottom of the microcapsule or cell to generate black or white surface visibility to an observer. Specifically, the particles are oriented or translated by placing an electric field across the cell. The electric field typically includes a direct current field, which may be provided by at least one pair of electrodes disposed adjacent to a display comprising the cell. Once set for a black state or a white state, the display maintains its color until a different configuration is forced through the application of a subsequent electrical field.
Such two-particle electrophoretic capsule systems, which have a contrast ratio of about 10 and a whiteness of more than 35%, are commercially available. However, these products are limited to small area displays, such as a personal digital assistants (PDAs) or handheld E-book, because a uniform close-pack coating of spherical capsules over large area is difficult to achieve where spherical capsules are deformed into a closed packed layer. Also, the electrophoretic ink systems cannot tolerate outdoor environmental conditions.
Such electrophoretic displays are disclosed, for example, in U.S. Patent Application Publication US 2004/0119680 to Daniel et al., which describes the switching of a two-particle electrophoretic display comprising two-particle electrophoretic ink consisting of a first particle species of a first color (e.g. white) and a second particle species of a second color (e.g. black) suspended in a clear medium. The disclosure of Daniel is incorporated herein in its entirety. The different colored particles of Daniel carry opposite charges, and the charged particles are moved by DC current application to change the electrophoretic display.
These two-particle electrophoretic systems are well recognized as providing good black and white electronic displays, but are currently limited by difficulties in making large area display media.
Thus, there remains a need for electrophoretic displays having good optical performance at high and low resolutions. There also remains a need for electrophoretic displays that can be used for large area and/or outdoor visual displays.